The present invention relates to a magnetic bubble memory device (hereinafter referred to as "bubble memory"). More particularly, it relates to a geometry of a pattern of magnetically soft elements defining a bubble propagation path in the memory device.
Conventionally, a bubble propagation path in the bubble memory is usually defined by the so-called gap-tolerant half disk (refer to FIG. 1A) or asymmetric chevron-shaped pattern for the magnetically soft elements. However, where it is intended to increase the density of the pattern, to cope with requirements for an increase in capacity of the memory and a reduction in size of the memory device, the analogous reduction in geometry of the gap-tolerant pattern is limited to a pattern period (i.e., propagation period) as small as 6 .mu.m, due mainly to the limitations stemming from forming the gap between the pattern elements by photolithography. Accordingly, a memory chip of, for example, 4 Mbits, is about 15 mm square in size and is larger than a 1 Mbit memory chip about 10 mm square in size. This results in problems arising such as an increase in drive power, and an increase in cost due to the lower number of memory chips that can be manufactured from a wafer.
Recently, a new bubble propagation pattern (refer to FIG. 2A), called a "wide-gap pattern", was reported by A. H. Bobeck, et al. (EA-1, 3M Conference, Atlanta, 1981). The wide-gap pattern comprises magnetically soft elements, each of which has an asymmetrically crooked or curved shape with an entrance-side segment and an exit-side segment having different lengths. These elements are arranged at a predetermined period so that the exit-side segment of one adjacent pattern element is opposed to the outer side of the entrance-side segment of the other pattern element, with a gap therebetween. According to a conventional half-disk pattern, as illustrated in FIGS. 1A and 1B, a magnetic bubble Bu is stretched at a wide and deep potential well PW produced in the gap region between the exit-side leg 1b of a pattern element 1 and the entrance-side leg 2a of a pattern element 2 opposite to each other, with a gap G therebetween, and is propagated from the pattern element 1 to the pattern 2. On the contrary, according to the wide-gap pattern, as illustrated in FIGS. 2A and 2B, a bubble Bu is propagated from a pattern element 3 to a pattern element 4 along the field gradient or potential gradient FG produced in the gap region between the exit-side segment 3b of the pattern element 3 and the entrance segment 4a of the pattern element 4, opposed to each other with a gap G therebetween. The gap tolerance of the wide-gap pattern is about twice that of the half-disk pattern. In fact, it has been proved that a wide-gap pattern for the propagation of about 2-.mu.m diameter bubbles having a pattern period P=8 .mu.m and a gap width G=2 .mu.m provides a bias margin of 30 Oe at the triangular drive field of 40 Oe, which is about twice that of the half-disk pattern having the same pattern period and gap width. Accordingly, the wide-gap pattern is favored where an increase in density is desired, and ways of making this wide-gap pattern fit for practical use have been proposed in, for example, U.S. Pat. No. 4,355,373.
In known wide-gap patterns, however, there is a problem that, where the pattern geometry is reduced in order to achieve a 4 Mbit memory chip, having the pattern period P=4 .mu.m and the gap width G=1 .mu.m, the potential in the gap region is very shallow and, as a result, the movement of the bubble at the beginning of the propagation thereof from the pattern element 3 to the pattern element 4 is very unstable and a satisfactory propagation characteristic cannot be ensured.
Further, for the arrangement of the bubble propagation path in the bubble memory device, a single loop (or serial loop) arrangement and a major-minor loop arrangement are known in the prior art (refer to FIGS. 11 and 12). In these arrangements, the bubble propagation path usually has an outbound track and a return track, which are arranged in parallel to each other and opposite in the direction of bubble propagation. In particular, the outbound track and the return track are indispensable for forming the minor loop in the major-minor loop arrangement. However, as described hereinafter, where the outbound track and the return track of the bubble propagation path are defined by, for example, the wide-gap patterns having the same geometry, the propagation characteristics of the outbound track and the return track are different from each other, due to the magnetic anisotropy of the bubble crystal and, accordingly, the propagation characteristic of the entire propagation path is defined by the worse track among the outbound tracks and the return tracks, thereby resulting in unsatisfactory propagation characteristics.